Remote supply circuit for analog two-wire connection

ABSTRACT

This circuit comprises at least one power supply inserted between the earth and each of the two wires of the connection, two RC series dipoles in each case inserted between the earth and the two wires and a circuit able to fix to the earth the average potential of these wires. 
     Application to telephony and the supply of sensors.

The present invention relates to a remote supply circuit for an analogtwo-wire connection. It is more particularly used in telephony, e.g. inthe production of automatic switches and more generally in the remotesupply of probes or sensors.

A remote supply circuit for an analog two-wire connection comprises inper se known manner and in accordance with the diagram of FIG. 1, asource 10 of voltage e, which represents information to be transmittedto a load 20, two voltage amplifiers 11 and 12 of respective gains -1and +1 and two identical impedances 13 and 14, said amplifiers andimpedances being inserted between load 20 and each of the wires 15, 16of the connection.

This circuit arrangement provides protection against interference. Thus,the analog data is transmitted in the form of a voltage differencebetween the two wires 15, 16, each having the same impedance relative toearth. Under these conditions, interference causes on the voltages e₁and e₂ of these wires substantially identical errors, which cancel oneanother out during the calculation of the difference e₁ -e₂. Thecontinuous remote supply is supplied by the direct current carried bythe two-wire connection.

In accordance with the diagram of FIG. 2, the transformer is the mostused component in this circuit arrangement. Transformer 22 comprises aprimary winding 22/1 connected to source 10 and a secondary winding22/2, constituted by two oppositely directed half-windings connected onthe one hand to a source 30 of d.c. voltage E providing the continuousremote supply and on the other hand to wires 15, 16.

In this application, the transformer offers numerous advantages:

it is in the form of a single component and not an assembly;

it is very robust with respect to overloads;

the pure resistance of its windings can be reduced;

it provides a good galvanic insulation.

However, it has the following disadvantages:

its volume is a rising function of the remote supply current (themagnetic circuit must not be saturated);

to ensure that the impedances of the two secondary half-windings are asclose together as possible they must be wound together (so-called twowires in hand winding system);

its volume is a rising function of the low cut-off frequency (the lowerthis frequency, the higher must be the inductances);

in addition, the lower the winding resistance the greater its volume;

finally, its cost is high.

These disadvantages outweigh the advantages, so that research has beencarried out on entirely electronic circuit arrangements to attempt toobviate the use of the transformer. As a function of the envisagedcircuits, the galvanic insulation is provided by means of opticalcouplers or by means of capacitors. In certain cases, this insulation isnot ensured on knowing the extreme voltages liable to occur on the wiresof the line. This is the case, for example, when the wires of the linehave protections such as Zender diodes, spark gaps or the like.

FIG. 3 illustrates a solution of this type. The circuit comprises twoamplifiers 31, 32, two resistors 33, 34 of value R₁, the line resistorsbeing symbolized by the two resistors 35, 36 of value R₂. The amplifiers31 and 32 supply signals of form: ##EQU1##

The difference E₀ =E₁₀ -E₂₀ represents the d.c. remote supply voltage,whilst e represents the analog signal transmitted. Furthermore, ondesignating by I₀ and U₀, the d.c. current and voltage required by theremotely supplied device 20, with i and u the variable current andvoltage constituting the analog data support transmitted to the saiddevice, we obtain:

    U=U.sub.0 +u  I=I.sub.0 +i

Under these conditions, it is possible to write:

    E.sub.1 -E.sub.2 =U+2R.sub.2 I+2R.sub.1 I

whence

    E.sub.0 =e=U.sub.0 +2R.sub.2 I.sub.0 +2R.sub.1 I.sub.0 +u+2R.sub.1 i+2R.sub.2 i

The above equation can be divided into two parts:

a remote supply equation:

    E.sub.0 =U.sub.0 +2R.sub.2 I.sub.0 +2R.sub.1 I.sub.0       ( 1)

a transmission equation:

    e=u+2R.sub.1 i+2R.sub.2 i                                  (2)

The remote supply equation reveals the disadvantage of the circuit ofFIG. 3, namely the voltage drop 2R₁ I₀ cuts the remote supply voltage E₀into a pure loss. To reduce this effect, it is necessary to reduce theremote supply current, which is not always possible.

The present invention aims at obviating the disadvantage, whilstproposing an electronic circuit of the type illustrated in FIG. 3.

The basic idea of the invention is to modify the circuit diagram of FIG.3 by adding to it two supplementary power supplies, carrying thereferences 37, 38 in FIG. 4, whilst the currents are supplied inopposite directions and with the intensity I₁.

Using the same notations as hereinbefore, the equations of this circuitarrangement are then written:

    E.sub.0 =U.sub.0 =2R.sub.2 I.sub.0 +2R.sub.1 (I.sub.0 -I.sub.1)(3)

    e=u+2R.sub.1 i+2R.sub.2 i                                  (4)

Equation (3) shows that if I₁ is equal to I₀, the remote supply voltageE₀ is not cut in the aforementioned manner and assumes the value U₀ +2R₂I₀.

Under practical conditions, the use of this principle involvesbalancing, control and stability problems, which are solved by thecircuit arrangement proposed by the invention. Firstly, a transformationknown under the name of the Thevenin-Norton transformation makes itpossible to pass from the diagram of FIG. 4 to that of FIG. 5. Thecircuit shown in the latter comprises two power supplies 41, 42 ofrespective values E₁₀ /R₁ and E₂₀ /R₁, two power supplies 43, 44 ofvalue 1/2 e/R₁, two resistors 33, 34 of value R₁ and two power supplies45, 46 of value I₀. A current I circulates in wires 15, 16, thedirection in which the currents flow is shown in the drawing.

On accepting the presence of a low cut-off frequency in the transmissionequation, it is possible to adopt a somewhat simpler circuit arrangementas shown in FIG. 6 using, in series with resistors 33, 34, twocapacitors 47, 48. It also makes it possible to eliminate the two powersupplies 41, 42, whose values are linked with the line resistance R₂(consequence of equation (3)). This leads to a remote supply in currentI₀ and not in voltage E₀ as therefore in the case of the prior art.

However, a disadvantage of this circuit is its lack of stability. Thus,the line voltages U₁ and U₂ are indefinite, which in practice leads tothe saturation of the stages realizing the power supplies. To fix theaverage potential of the line wires, it is necessary to add a controldevice able to fix to earth the average potential of the line wires.This is the circuit adopted for the present invention.

More specifically, the present invention relates to a remote supplycircuit for an analog two-wire connection comprising at least onecurrent source inserted between the earth and each of the two wires ofthe connection, wherein the said source comprises a direct currentremote supply source and a variable transmission current source andwherein it comprises two RC series dipoles in each case inserted betweenthe earth and the said two wires and a circuit able to fix to the earththe average potential of the wires.

The invention is described in greater detail hereinafter relative tonon-limitative embodiments. This description refers to the drawingsfollowing the aforementioned FIGS. 1 to 6 and in which show: FIG. 7 theblock diagram of the circuit according to the invention. FIG. 8 aparticular embodiment of this circuit. FIG. 9 a variant of the circuitfor fixing the average potential of the lines.

The circuit shown in FIG. 7 comprises a double current source 50constituted by a source 51 of remote supply direct current I₀ and asource 52 of variable transmission current i, two RC series dipoles 61,62 and a circuit 70 able to fix to the earth the average potential ofwires 15 and 16 of the connection. This is the general circuit diagramof the invention. Special embodiments of the different functional blocksused will now be described in conjunction with FIGS. 8 and 9.

In the circuit arrangement illustrated in FIG. 8, the circuit 50 isformed by a voltage source 53 of continuous value E and a voltage source54 of variable value e. E is the control voltage of the remote supplycurrent and e that of the transmission current. These sources areconnected to the bases of two transistors T₁ and T₂, whose emitters areconnected by a resistor R₈ and whose collectors are connected to twoamplification circuits constituted by a differential amplifier (A₁, A₂),whose output drives the base of a transistor (T₃, T₄), whose collectoris collected to one of the two wires of the line.

Transistors T₁, T₃ and T₂, T₄ and amplifiers A₁, A₂ are supplied by twolines 55, 56 at d.c. voltages +V and -V. These lines 55, 56 areconnected to transistors T₁, T₂ by resistors R₇, to amplifiers A₁, A₂ byZener diodes Z₁, Z₂ and transistors T₃, T₄ by resistors R₆. Moreover,diodes Z₁, Z₂ are interconnected by a resistor R₉.

The RC dipoles 61, 62 are formed by means of two resistors R₁ and R₅connected by an amplifier A₄, A₅ and by a capacitor C₁. This circuitarrangement has the advantage of reducing the value and therefore thesize of capacitor C₁ for a given time constant. Calculation shows thatthe thus synthesized dipole is equivalent to a resistor of value R₅, R₁in series with a capacitor of value (1+R₅ /R₁)C₁.

Control circuit 70, which makes it possible to fix the average potentialof the line wires to earth comprises in the illustrated variant, adifferential amplifier A₃, whose negative input is connected to thecentre of two resistors of value R₃, the positive input is connected toearth and the output is connected to the centre of two resistors R₄,which are also joined to the two wires 15, 16 of the line. This circuitmakes the impedance in the common mode of the line wires low comparedwith the earth and does not act on the symmetrical differential mode. Avariant of this circuit arrangement is given in FIG. 9, which uses twotransistors T₆, T₇. The injection of control currents need not takeplace on the line wires and can instead take place directly upstream andin parallel on the current sources.

The diagram of FIG. 8 contains other elements and their function willnow be described. A circuit 80 comprises an amplifier A₆ and twocapacitors C₂ connected to the line wires 15, 16. Amplifier A₆ ismounted in differential and serves to extract the differential signalfrom the line. The two capacitors C₂ rejects the continuous component ofthe differential signal.

A circuit 82 essentially comprises an amplifier A₇ and connected as asummator and which serves to remove from the line differential signalthe transmitted differential signal so that the voltage s which itsupplies is only the image of the differential signal received from theremotely supplied device 20. With this objective, the representedcircuit comprises a circuit 86 for compensating the transmitted signal.The circuit 84 is associated with an amplifier A₈, which serves tosynthesize the necessary transfer function, which is dependent on theimpedance Z of the remotely supplied device in the useful band.

Calculation shows that the transmission equation is written as a firstapproximation: ##EQU2## in which R₇ and R₈ are resistances of thecurrent sources of circuit 50, R₁ the resistance of dipoles 61 and 62and R₂ the line resistance. If Z is real, then so is the associatedtransfer function.

Finally, an amplifier A₉ connected as a comparator and a transistor T₉at the output serve to establish whether or not transistor T₃ issaturated. Thus, they make it possible to know whether the remotelysupplied device is or is not operating correctly and also make itpossible to detect the presence or absence of this device, i.e. if theline is open T₄ is saturated.

The illustrated circuit is completed by a number of non-essentialelements such as two diodes D₁, D₂ positioned between supply lines 55,56 and line wires 15, 16 and two Zener diodes Z₃, Z₄ between the wiresand the earth.

Thus, it is apparent that the circuit according to the invention offersnumerous advantages:

there are separate controls for the remote supply current and thetransmission current;

the transmission and remote supply characteristics are independent ofthe supplies +V and -V;

the only condition regarding +V and -V is the saturation of transistorsT₃ and T₄ making it possible to use supplies which can be cut off by amicroprocessor and which are automatically adjusted to the optimumvalue, no matter what the terminal connected or the line resistance R₂ ;

the absence of a high value capacitor makes it possible to construct thecircuit arrangement on the basis of hybrid technology;

all the amplifiers (except A₄ and A₅) are at low voltage and current,i.e. inexpensive;

the remote supply current can easily be controlled by a microprocessor;

the transfer function used for compensation can also be adjusted by amicroprocessor using digital synthesis.

I claim:
 1. A remote supply circuit for an analog two-wire connectioncomprising at least one current source inserted between the earth andeach of the two wires of the connection, wherein the said sourcecomprises a direct current remote supply source and a variabletransmission current source and wherein it comprises two RC seriesdipoles in each case inserted between the earth and the said two wiresand a circuit able to fix to the earth the average potential of thewires.
 2. A circuit according to claim 1, wherein the current sourcecomprises a remote supply voltage source connected to the base of afirst transistor and a transmission voltage source connected to the baseof a second transistor, the emitters of these two transistors beinginterconnected by a resistor and the collectors are connected to the twoline wires by an amplifier connected to the base of a furthertransistor.
 3. A circuit according to claim 1, wherein the circuit ableto fix the average potential of the two line wires to the earthcomprises a differential amplifier having a positive input connected tothe two wires by two equal resistors and a negative input connected tothe earth and having an output connected to the two wires by two equalresistors.
 4. A circuit according to claim 1, wherein each RC dipole issynthesized by means of two resistors connected by an amplifier, theassembly being in series with a capacitor.